Conventionally, for example, a pneumatic tire as described in Japanese Unexamined Patent Application Publication No. H6-55912A that aims to reduce tire noise, particularly patterned noise, and to provide excellent water drainage is known. This pneumatic tire includes a plurality of land parts in a tire tread that are continuous in a circumferential direction of the tread, being defined by a tread end and a plurality of main grooves extending along a circumference of the tread and arranged with spacing in a tread width direction. In this pneumatic tire, sipes in the tread extend from each tread end and pass obliquely through the main grooves toward a convergence at a central region of the tread, being arranged with substantially equal spacing in the circumferential direction of the tread. The sipes intermittently include groove-like enlarged portions with an enlarged opening width in a tread surface.
Also, for example, a pneumatic tire as described in Japanese Unexamined Patent Application Publication No. 2011-51408A that aims to improve snow performance without affecting braking performance on a dry road surface is known. In this pneumatic tire, a tread pattern is based on a plurality of blocks defined and formed by a plurality of circumferential grooves provided in a rotation direction of the tire and a plurality of lateral grooves provided in a width direction of the tire. Among the blocks, a block arranged on a shoulder side is provided with a lug groove that passes through a side wall of the block and has one end thereof opening to one of the circumferential grooves.
In addition, for example, a pneumatic tire as described in Japanese Unexamined Patent Application Publication No. 2013-39899A that aims to improve performance on snow while maintaining wet performance is known. In this pneumatic tire, a surface of a tread of the tire includes a plurality of circumferential grooves extending in a tread circumferential line and land parts defined by the circumferential grooves. In at least one of the land parts, a plurality of lateral grooves and lateral sipes are formed to extend from one of two circumferential grooves that define the one of the land parts to the other one, and to open to at least one of the circumferential grooves. On both sides of at least one of the land parts, a plurality of protrusions are formed so as to protrude from a bottom of the two circumferential grooves that define the land part to an outer side in a tire radial direction, and to be continuous with a side wall of the land part.
Furthermore, for example, a pneumatic tire as described in Japanese Unexamined Patent Application Publication No. 2010-168006A that aims to improve dry performance while maintaining snow performance at a conventional level is known. In this pneumatic tire, a tread surface has five land parts defined and formed by four main grooves extending in a tire circumferential direction. The five land parts include a center land part, two middle land parts on outer sides of the center land part in a tire width direction, and two shoulder land parts further outward in the tire width direction. The land parts include sub grooves and sipes. Total groove surface area ratios of the sub grooves and the sipes in the land parts vary among the center land part, the shoulder land parts, and the middle land parts such that the center land part and the shoulder land parts have a lower ratio and the middle land parts have a higher ratio.
In the pneumatic tires of Japanese Unexamined Patent Application Publication Nos. H6-55912A, 2011-51408A, 2013-39899A and 2010-168006A as described above, and mainly in Japanese Unexamined Patent Application Publication No. 2010-168006A, a tread surface has five land parts defined and formed by four main grooves extending in a tire circumferential direction. The five land parts include a center land part, two intermediate land parts on outer sides of the center land part in a tire width direction, and two shoulder land parts further outward in the tire width direction. Also, sub grooves and sipes are formed in each of the land parts. The sub grooves and the sipes have drainage characteristics for water and for snow, and provide an improvement to performance on snow (snow performance). However, the sub-grooves and the sipes cause a decrease in block rigidity, and tend to decrease steering stability performance on dry roads (dry performance).
In Japanese Unexamined Patent Application Publication No. 2010-168006A, for dry road surfaces, a groove surface area ratio is decreased in the center land part, which affects steering stability performance while driving straight, and in the shoulder land parts, which affect steering stability performance while cornering. This enhances the rigidity of the center land part and the shoulder land parts, and serves to improve the steering stability performance on dry road surfaces. However, the steering stability performance on dry roads is highly affected by the middle land parts. The rigidity of the middle land parts is decreased due to that the middle land parts are formed in a block shape, which makes improvements to the steering stability performance on dry roads remarkably difficult to obtain.